According to the state of the art, latex foam is manufactured by a procedure in which a composition which is foamable, gelable and heat-vulcanizable and comprises an aqueous rubber dispersion, a sulfur vulcanizing agent and conventional additives, is foamed with air or another gas and is mixed and gelled with a gelling agent such as for example sodium silicofluoride (Na.sub.2 SiF.sub.6), and is then vulcanized under hot conditions. Gelling takes place at room temperature, in an infrared zone or during the heating-up phase preceding vulcanization. The latex foam so manufactured comprises a cellular structure (E. W. Madge, Latex Foam Rubber, Mclaren & Sons, London, 1962).
Latex foam is desired to have a pattern of properties which may be defined by the following combination:
(a) the hardness, i.e., the indentation hardness C (DIN 53 576) must be sufficiently high;
(b) the pore structure is to be uniformly fine for an adequate layer thickness.
A further requirement is reliability, more particularly the assurance that in the manufacture of the latex foam from a wet foam no processing problems, i.e., in particular no gelling problems are to occur.
Further properties of the latex foam which may optionally be desired depending on the intended use are:
(c) high elasticity (flexibility), i.e, in the desired temperature range, any residual compressive deformation after prolonged compression (permanent deformation, compression set) is to be as low as possible (DIN 53 572);
(d) high damping in the desired temperature range; or
(e) a good compromise of adequately high elasticity and adequately high damping in the desired temperature range; in this context long-term compression fatigue should be sufficiently low (prolonged vibration test, DIN 53 574).
The hardness is controlled essentially by the mass to volume ratio of the latex foam (the lower the degree of foaming and the higher the corresponding mass to volume ratio, the higher will be the indentation hardness C) and by the selection of the rubber component. The latter is either a non-reinforced or a reinforced rubber component. In the latter case, the lowest possible mass to volume ratio for a given indentation hardness C is aimed at, and high cost effectiveness is attained, a reduction of elasticity due to the reinforcement being tolerated (DE-PS 10 56 364; DE-OSS 14 70 810 and 34 47 585).
The above described combination of properties (a and b) is, generally speaking, attained adequately in forms of applications which require an indentation hardness C (at 40% deformation) of less than 400N (typical mass to volume ratio 0.1 g/cm.sup.3, light foam) or which, at only 6% deformation, similarly require a value C of less than 400N (typical mass to volume ratio of 0.3 g/cm.sup.3, bordering on heavy foams).
However, this is not attained if latex foams with indentation hardnesses C at 6% deformation of more than 500N, in particular more than 1000N, are required, especially for foams of a layer thickness of more than 10 mm. Such latex foams cannot be produced from wet foams comprising a non-reinforced or reinforced rubber component. At mass to volume ratios of more than 0.3 g/cm.sup.3, gelling problems were usually experienced, in particular, at layer thicknesses of more than 10 mm. The latex foams so attained suffered from an uneven pore structure and from surface blemishes. They were accordingly useless. A slight improvement of the pore structure was attainable by the addition of chalk as a filler (20 pphr, comparative example A; see Table). However, this expedient resulted in impaired elasticity.